Pressure tank, in particular hydraulic accumulator

ABSTRACT

In a pressure tank, in particular hydraulic accumulator ( 3, 5 ), having a parting element ( 1 ), which parting element ( 1 ) separates a space ( 11 ) for a first, in particular gaseous, working medium from a space for a second working medium, in particular a fluid, in the tank, is flexible, can move under deformation and defines a domed main parting plane which extends from an annular edge ( 13 ), the parting element ( 1 ) is produced from a substance which has a fluoroplastic material, preferably a substance composed entirely of fluoroplastic material.

The invention relates to pressure tank, in particular to a hydraulic accumulator, with a resilient separating element which can be moved with deformation and which separates a space in the tank for a first, in particular gaseous working medium, from a space for a second working medium, in particular a fluid, and which defines a domed main separating plane which extends from an annular edge.

A pressure tank of this type in the form of a hydraulic accumulator is disclosed in DE 28 52 912 A1. The resilient separating element which consists of a rubber-like material (synthetic rubber, such as acrylic nitrile-butadiene rubber), in the known hydraulic accumulator forms a membrane which can be moved by deformation and which separates the gas side from the liquid side in the accumulator housing. Two main demands must be imposed on the operating behavior of hydraulic accumulators with these membranes which can be moved by deformation. On the one hand, the impermeability of the membrane must be ensured to prevent gas diffusion. On the other hand, corresponding mechanical properties of the membrane are necessary, especially ease of movement and high cyclic bending strength which are maintained even under the influence of corrosive media.

In the aforementioned, known hydraulic accumulator these requirements are only partially satisfied. In order to improve the impermeability of the rubber-like membrane, in the known accumulator there are annular bead-like elevations which project out of the main separating plane in tight succession. Because the elevations increase the average wall density, diffusion tightness is in fact improved, but the significant increase of wall thickness leads to considerable stiffening and accordingly to a deterioration of mobility.

With respect to this prior art, the object of the invention is to make available a pressure tank, in particular a hydraulic accumulator, that is characterized by much improved operating behavior in comparison.

According to the invention, this object is achieved by a pressure tank which has the features of claim 1 in its entirety.

In that, according to the invention, in the pressure tank there is a separating element which is produced from a substance which has a fluoroplastic material or consists preferably entirely of fluoroplastic material, on the one hand outstanding diffusion tightness is ensured, while on the other hand a separating element is provided which has mechanical properties that are optimum for use as a membrane in hydraulic accumulators, such as extreme cyclic bending strength. Therefore, very small wall thicknesses can be used; this leads to the desired ease of movement of the membrane. Based on the resulting good response behavior, the pressure tank is therefore especially well-suited for use as a pulsation damper.

Polytetrafluoroethylene has been found to be an especially suitable material.

Polytetrafluoroethylene (PTFE) due to its very high melt viscosity cannot be plastically molded, and the desired molded article from this material is cold pressed from powdered raw material with 200 to 400 bar and is sintered unpressurized at 370° to 380°. If films are to be obtained they are generally peeled off solid cylindrical blocks. Polytetrafluoroethylene therefore is commercially available in general in the form of rigid solid bodies such as slabs, rods, tubes, etc. For one with average skill in the art in the field of membrane technology it is surprising that he can nevertheless obtain separating elements which are produced in whole or in part from polytetrafluoroethylene material and which have high mobility such that they can even assume the function of a flexible rolling membrane.

Since PTFE materials can moreover have especially good chemical resistance, the pressure tank according to the invention is also suitable for use in the presence of chemically corrosive media.

In advantageous embodiments the separating element defines a domed main separating plane on whose side, which lies inside relative to the dome, annular bead-like elevations are made projecting. By using a membrane which is domed in this way, in the pressure tank a separating wall with a comparatively large area is available which, with ease of deformation, can effect a comparatively large change of volume of the bordering working spaces in the pressure tank.

In preferred embodiments, succeeding elevations are separated from one another by flat wall sections which extend along the main separating plane. Therefore, between adjacent elevations there is one free space at a time which is available for relative movements of adjacent elevations so that without annular beads which border one another mutually supporting one another and stiffening the structure, the separating element can undergo deformation as a rolling membrane.

Preferably, the peaks of the annular bead-like elevations have a round dome so that notch effects are avoided.

In especially advantageous embodiments the annular bead-like elevations are formed by folds which are open on the outer side and form annular groove-like depressions in the main separating plane here. According to the height of the folds, in a membrane which is made in this way, similarly to the case of a bellows, an especially great length of the material strip which can be moved is available in order to roll up or pull out the membrane.

Preferably, the arrangement in this instance is made such that the height of at least one fold measured from the open end to the peak of the folds along its vertical axis is different relative to the height of other folds.

As has been found, especially good mechanical properties are obtained when the first fold nearest the annular edge has a smaller height than the other adjoining folds.

In this respect, it is also advantageous if the wall section which extends from the annular edge to the nearest first fold has a wall thickness which on the annular edge has the largest value and decreases toward the first fold to the value of the wall thickness of the wall sections between the folds. The edge thickening formed in this way, without adversely affecting the resilience of the remaining membrane, promotes the clamping of the membrane on the assigned housing element of the pressure tank and the formation of a seal connection at the clamping site.

The invention is explained in detail below using the drawings.

FIG. 1 shows a cutaway and slightly schematically simplified longitudinal section of one exemplary embodiment of the pressure tank according to the invention in the form of a hydraulic accumulator, only the region of the bottom part of the housing and the bordering part of the top part of the housing being shown;

FIG. 2 shows a longitudinal section of only the separating element of the exemplary embodiment from FIG. 1, which element is made as a rolling membrane, and which section is shown as one half side and enlarged compared to FIG. 1, and

FIG. 3 shows a partial view of the region identified with III in FIG. 2 which has been further enlarged compared to FIG. 2.

Of the exemplary embodiment of the pressure tank according to the invention in the form of a hydraulic accumulator, FIG. 1 shows merely the bottom part 3 of the housing with a bottom-side fluid connection 9 which is concentric to the longitudinal axis 7 of the housing, and a piece of the top part 5 of the housing which borders the bottom part 3 of the housing. At the connection site between the bottom part 3 of the housing and the top part 5 of the housing the open, annular edge 13 of the separating element is clamped tight in the form of a rolling membrane which is designated as a whole as 1. Here the thickened edge 21 of the rolling membrane 1 on the one hand is supported on an annular surface 22 of the bottom part 3 of the housing and on the other hand adjoins an O-ring 15 which in turn sits in an annular groove 20 on an axially projecting annular body 14 of the top part 5 of the housing.

FIGS. 1 and 2 show the roll membrane 1 in the completely unrolled or extended state, in which the space 11 located above the membrane 1 in FIG. 1, the gas side of the hydraulic accumulator has the largest volume and there is no fluid pressure at the fluid connection 9 so that the membrane 1 lies against the inside wall of the bottom part 3 of the housing, a central reinforcing bead 29 of the membrane 1 overlapping the edge of the fluid connection 9 and in this way forming a mechanical safeguard against the membrane 1 being pressed into the fluid connection 9 when fluid pressure is absent.

FIGS. 2 and 3 illustrate more details of the rolling membrane 1 produced from PTFE material. Due to the very good diffusion tightness of the PTFE material and especially good strength properties, for the rolling membrane 1 merely a small wall thickness of the membrane as is emerges from the annular edge 13 is necessary; this defines the domed main separating plane. Successive annular bead-like elevations project to the inside from this main separating plane and are formed in the illustrated example, not by beads in the form of solid bodies, but by folds, of which the first fold nearest the edge 13 is designated as 17 and the adjoining folds are each designated as 19. As is apparent from FIG. 1, proceeding from the thickened wall 21 on the annular edge 13 the wall thickness changes such that the wall thickness decreases as far as the first fold 17 to the thickness value of flat wall sections 23 which are each located between the folds 17 and 19. In a practical embodiment the wall thickness decreases from the thickening 21 to the first fold 18 from a value of 1.2 mm to a value of 0.5 mm which is given on the succeeding wall section 23 between the folds 17 and 19. As FIG. 2 likewise shows, the thickened edge 21 on the inside forms a type of shell shape which forms a partial enclosure of the O-ring 15 which is not shown in FIG. 2.

As can likewise be recognized from FIG. 2, the fold height which is measured along the vertical axis 25 for the first fold 17 is smaller than for the succeeding folds 19 which each have the same height, all folds 17 and 19 being domed at their peak. The folds 17 and 19 are open on the side which is the outer side relative to the dome, so that annular groove-like depressions 27 (see in particular FIG. 3) are formed which each form interruptions in the course of the dome of the main separating plane between the wall sections 23. As can be recognized especially from FIG. 3, the inside width of the annular groove-like depressions 27 on the open end of the folds 17, 19 is much smaller than the fold height measured along the vertical axis 25, in this example the height of the folds 19 being larger approximately by a factor of 4.

As is likewise apparent from FIG. 3, the insides of the depressions 27 of the folds 17, 19 extend slightly diverging toward the open end so that the open end of the depressions 27 has a greater width than the base of the depressions 27 on the peak region of the folds. As FIG. 2 shows, the vertical axes 25 of the folds 19 each run in roughly the vertical direction to the tangential plane to the adjacent wall sections 23, while the vertical axis 25 of the first fold 17 extends slightly tilted to this tangential plane, the vertical axis 25 of the first fold 17 enclosing an angle of approximately 10° with the plane of the annular edge 13. For the succeeding folds 19 the vertical axes 25 from fold to fold are tilted increasingly more steeply to the plane of the edge 13.

In this example, the annular bead-like elevations projecting on the inside of the membrane 1 are formed by folds 17 and 19, as a result of which especially easy mobility for rolling up the membrane results. But there could also be annular bead-shaped elevations made as solid bodies. Unfilled PTFE materials can be used, or those with a filler and/or filler combinations as can be provided conventionally for PTFE materials; for example, when extreme temperature resistance or other special properties are desirable. Glass fiber materials, carbon, or metallic fillers can be considered, among other materials.

Semifinished articles of PTFE materials are available in many forms, for example, films peeled off blocks, solid bars, round blanks, and the like. Based on the mechanical properties, finished products, such as the rolling membrane used in the pressure tank according to the invention, can be produced by cutting from molded bodies; these bodies for their part are pressed and sintered from powdered raw material. In particular, for thin-walled articles, however, shaping by blow molding of a PTFE dispersion before sintering is possible. If the spherical membrane shape shown in FIG. 1 is obtained from a solid polytetrafluoroethylene body, it can then be brought into the illustrated shape of the separating membrane by cutting of the raw body. In order to minimize the polytetrafluoroethylene scrap which forms in the cutting process, preferably a preform body as the blank can be produced in a half shell shape as a mold.

The indicated polytetrafluoroethylene material as a fluoroplastic material can comprise both pure PTFE and also modified PTFE and can include both unfilled PTFE and also PTFE compounds. For a modified PTFE material, fillers such as bronze, carbon dust, MoS₂, as well as glass fiber and carbon fiber materials are possible. In addition to PTFE, as other fluoroplastic materials the following can be used: ethylene tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethylene copolymer (ECTFE), polychlorotrifluoroethylene copolymer (PCTFE), perfluoroalkoxy copolymer (PFA), polyvinylidene fluoride (PVDF) and tetrafluoroethylene perfluoropropylene (FEP). 

1. A pressure tank, in particular a hydraulic accumulator (3, 5), with a resilient separating element (1) which can be moved with deformation and which separates a space (11) in the tank for a first, in particular gaseous working medium, from a space for a second working medium, in particular a fluid, and which defines a domed main separating plane which extends from an annular edge (13), characterized in that the separating element (1) is produced from a substance which has a fluoroplastic material, preferably consists entirely of fluoroplastic material.
 2. The pressure tank according to claim 1, characterized in that the separating element (1) defines a domed main separating plane on whose side which lies inside relative to the dome annular bead-like elevations (17, 19) are made projecting.
 3. The pressure tank according to claim 2, characterized in that succeeding elevations (17, 19) are separated from one another by flat wall sections (23) which extend along the main separating plane.
 4. The pressure tank according to claim 2, characterized in that the peaks of the annular bead-like elevations (17, 19) have a round dome.
 5. The pressure tank according to claim 2, characterized in that the annular bead-like elevations are formed by folds (17, 19) which are open on the outer side and form annular groove-like depressions (27) in the main separating plane here.
 6. The pressure tank according to claim 5, characterized in that the height of at least one fold (17) measured from the open end to the peak of the folds (17, 19) along its vertical axis (25) is different relative to the height of other folds (19).
 7. The pressure tank according to claim 6, characterized in that the first fold (17) nearest the annular edge (13) has a smaller height than the adjoining other folds (19).
 8. The pressure tank according to claim 6, characterized in that the height of the folds (17, 19) is larger at least by a factor of two than the width of the annular groove-like depressions (27) formed, which width is measured on the open end of the folds (17, 19).
 9. The pressure tank according to claim 6, characterized in that the vertical axes (25) of succeeding folds (17, 19) run tilted by a small angle to one another.
 10. The pressure tank according to claim 5, characterized in that the flat wall sections (23) which extend between successive folds (17, 19) each have the same wall thickness.
 11. The pressure tank according to claim 9, characterized in that the wall section which runs from the annular edge (13) to the nearest first fold (17) has a wall thickness which on the annular edge (13) has the largest value and decreases toward the first fold (17) to the value of the wall thickness of the wall sections (23) between the folds (17, 19). 